The inventions disclosed and taught herein relate generally to treatment fluids for use in treating subterranean zones and formations penetrated by well bores. In particular, the inventions relate to viscosified treatment fluids and compositions containing a stabilized breaker, and methods for using such fluids.
Carbohydrate polymers, crosslinked with various ions, such as boron, zirconium, and titanium, are used as high-viscosity fracturing fluids in the oil and gas industry. Polysaccharides, such as guar and guar derivatives, are commonly used as viscosifying water-based fluids for fracturing treatments and for proppant transport. The proppant remains in the produced fracture in order to keep the fracture open and create a conductive channel extending from the well bore into the formation along the fracture length. After the fracture is complete, the recovery of the fracturing fluid is crucial to accelerate hydrocarbon production through the formed channel.
The recovery of the fracturing fluid is achieved by reducing the viscosity of the fluid such that the fluid flows naturally through the proppant pack. Chemical reagents, such as oxidizers, acids and enzymes are typically employed to break the polymer networks to reduce their viscosity. These materials are commonly referred to as breakers.
The timing of the break is critical. Gels broken prematurely can cause proppant to settle out of the fluid before reaching a sufficient distance into the produced fracture and result in a premature screen-out. Premature breaking can also result in less desirable fracture width in the created fracture. On the other hand, too much delay in breaking the gel is not desirable either. Delayed breaking can cause significant reduction in the hydrocarbon production. These factors, including breaker reactivity level versus temperature, delay mechanisms, and insufficient clean-up of the proppant pack impose significant complexity in designing a successful breaker system.
Ammonium persulfate (APS), is one of the most widely used breakers in the industry. When APS is used, free sulfate radicals are generated due to thermal decomposition of the persulfate ions upon homolytic cleavage of the peroxo (O—O) bond. This free radical initiates a chain scission process by interacting with the polymer chain to abstract hydrogen, which results in the primary bond cleavage of either the mannose or galactose groups. The generated radicals propagate the process, further breaking the polymer into lower molecular weight fragments. This continues until the termination of the reaction occurs, mostly due to the combination of two radicals.
Although the use of APS breaker systems is widely accepted, the use of this type of breaker suffers from several drawbacks, including slow break times at temperatures below about 120° F., and fast break times for temperatures greater than 120° F. Consequently, other options have been used, including enzymatic breakers and peroxide breaker systems. The first of these, while often effective, can be expensive to use and are sometimes inefficient in a typical well treatment situation. The peroxide breaker systems, while both effective and cost efficient, have the inherent drawback of being classified as “oxidizers”, thus increasing costs in transport, and storage, and raising concerns of long term stability.
The inventions disclosed and taught herein are directed to well treatment compositions that include calcium peroxide (CaO2) particles which are manufactured so as to be classified as a non-oxidizer under standard testing methods, while maintaining oxidizing properties, including available oxygen content, suitable for their use as polymer breaking agents in hydrocarbon recovery operations.
For the purposes of the instant invention, the term “particles” means a powder or granule or multi-layer tablet (etc.) made of free particles, preferably with a low moisture content (typically below about 1%).